Possible changes in feedback sensitivity to estrogen during puberty will be examined by ovariectomizing female rats at either 23 or 60 days of age and treating them with graded doses of estradiol benzoate over the next 7 days; serum samples obtained at autopsy on day 8 will be assayed for LH content. In a second study, adult female rats will be exposed to 2, 8, 16 or 22 hours of light per day for about 6 weeks, then ovariectomized; groups will be sacrificed at weekly intervals to compare serum LH levels in the 4 photoperiods. Other ovariectomized animals will receive graded doses of estradiol benzoate, first in small daily doses to determine amounts required to suppress LH release, then in single larger doses to induce a "surge" of LH, comparable to the ovulatory surge. The purpose of this study is to test the possibility that changes in cycle length observed in different photoperiods are due to changes in either the negative or "positive" feedback effects of estrogen on LH release. In a third day, serum estrogen levels (and serum LH levels) will be measured by radioimmunoassay in animals that are in persistent estrus, either as a result of exposure to constant light or occurring spontaneously in older rats. The purpose of the study is to determine whether the origin of the persistent estrus in these two sets of circumstances is the same or different; in either case, it will allow a more accurate description of events responsible for maintenance of reproductive cyclicity.